"The penchant of parents in the US to invest in the higher able kids will aggravate initial differences between siblings of differing cognitive ability," says Frijters.

Frijters says experts have long debated whether parents tend to invest more in children who are clever over those who are falling behind, or the other way around.

However studies that try to compare the impacts of these so-called "Spartan" and "Samaratin" approaches to child rearing, face challenges, says Frijters.

How, for example, do you measure the natural cognitive abilities of children prior to any parental input?

To get around this problem, Frijters and colleagues used a well-known marker for natural cognitive ability - handedness, which is an attribute children are born with.

A proxy for cognitive ability

About 10 per cent of children are left-handed and the consensus is that they are less cognitively developed than right-handed children, says Frijters - although the jury is still out on why this is the case - perhaps it's because they are living in a 'right-handed world'.

By using handedness within families as a proxy for cognitive ability, the researchers could be sure that they were identifying the natural cognitive ability of the children in the study.

Using a large national longitudinal study of youth in the US, the researchers analysed data on 2318 children from 850 households.

Each household studied had at least one left-handed and one right-handed sibling.

Each child was given a battery of tests to assess their level of understanding of maths and English, their social development, interpersonal skills and their number of friends.

The parents in the study were asked to keep a diary on how much time they spent with their children, explaining things to them and helping them to read, and to record the money they spent on tutoring.

"We found that across these householders the right-handed kids got more attention than the left handed kids," says Frijters.

Since the cognitive ability measures and parental investment differs at different ages, the data had to be standardised.

"One standard deviation increase in child cognitive ability increased parental investment by about one third of a standard deviation," says Frijters.

He says the concern is that such differential attention from parents will aggravate differences between siblings.

This means smart kids will get smarter at the expense of the children with lower cognitive ability.

Australian study

Interestingly, when Frijters and colleagues did a related study in Australia some years ago, it suggested an opposite trend.

In that study, mothers of left-handed kids tended to stay at home more, suggesting the children with lower cognitive ability were getting greater attention from their parents.

Frijters speculates this is because the US is a more 'winner takes all' society.

"There's a much larger degree of income inequality in the US than there is in Australia," he says. "Betting on the cleverest kid is like taking out a ticket in a lottery that you might make it very rich."

Australia, by contrast has less income disparity and is more egalitarian and thus the incentives to invest in the cleverest child are lower, says Frijters.

He says while it's hard to prove the impact of such early influences on education and employment later in life but, they are likely to have a huge effect.

"If all you know about someone is their cognitive development when they were young, those that were more developed are likely to turn out better in every way measurable," says Frijters.